Chiral lanthanocene derivatives containing two linked amido-cyclopentadienyl ligands: Heterobimetallic structure and lactone polymerization activity

Reaction of 2 equiv of dilithium amido-cyclopentadienide Li-2(C5R4SiMe2NCH2CH2X) (C5R4 = C5Me4, C(5)H(3)tBu; X = OMe, NMe2) with anhydrous LnCl(3) (Ln = Y, Lu) gave C-2-symmetric complexes of the type Li[Ln(eta(5):eta(1)-C5R4SiMe2NCH2CH2X)(2)] containing a heterobimetallic core. The molecular structure of Li[Y(eta(5):eta(1)-C5Me4SiMe2NCH2CH2OMe)(2)] was determined by single-crystal X-ray structural analysis, which showed it to exhibit both a pseudotetrahedral yttrium as well as a tetrahedral lithium center. In accordance with a formal 20-electron configuration at the rare earth metal center Ln, the amido-nitrogen atoms are shown to be pyramidalized. In the case of Li[Y(eta(5):eta(1)-C(5)H(3)tBuSiMe(2)NCH(2)CH(2)X)(2)], mixtures of the two diastereomers in relative ratios depending on the reaction time are formed. The (R,S)-diastereomer is the kinetic product that rearranges to the thermodynamically preferred C-2-symmetric (R,R)-diastereomer in donor solvents such as THF. The molecular structures of diastereomerically pure (R,S)-Li[Y(eta(5):eta(1)-C(5)H(3)tBuSiMe(2)NCH(2)CH(2)OMe)(2)] and (R,R)-Li[Y(eta(5):eta(1)-C(5)H(3)tBuSiMe(2)NCH(2)CH(2)NMe(2))(2)], as determined by single-crystal X-ray structural analyses, confirm the significantly different steric congestion. All heterobimetallic complexes were shown to be active in the ring-opening polymerization of epsilon-caprolactone. The poly(epsilon-caprolactone)s obtained are of high molecular weight (M-n > 30 000) and moderate polydispersity (M-w/M-n < 2.0) and are produced by the action of the nucleophilic amido-nitrogen atom.